1. Field of the Invention
The present invention relates to an internal combustion engine, more particularly to a spark ignition engine of the direct cylinder injection type which directly injects fuel into combustion chambers of the engine and causes combustion by spark ignition and which purifies the exhaust from the combustion chambers of the engine by a catalytic converter.
2. Description of the Related Art
An example of a conventional direct cylinder injection type spark ignition engine is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-169834 (see U.S. Pat. No. 4,955,339). In this engine, fuel injectors driven by actuators using piezoelectric elements and having a fast response are mounted on the cylinder heads of the engine. Fuel such as gasoline raised to a high pressure by a fuel injection pump is directly injected into the combustion chambers of the engine at suitable times in the first half of the intake stroke of the engine and the second half of the compression stroke in respectively suitable amounts and is ignited by the spark plugs mounted on the cylinder heads and burned.
The spray of fuel injected in the second half of the compression stroke forms an easily ignitable air-fuel mixture layer of about the stoichiometric air-fuel ratio just around the spark plugs even in the case of a lean air-fuel mixture in the combustion chambers. This serves as a pilot flame so that the flame propagates to the lean layer. This engine does not, like a diesel engine, in principle use a throttle valve to throttle the amount of intake for control of the output, so the combustion is performed under a three- to five-fold richness of air. Therefore, even during idling and other low load states, a large amount of air is taken into the combustion chambers, the air-fuel mixture becomes lean. Ignition is not possible by the spark plugs in that state, so stratified charge combustion is performed by the means as mentioned earlier.
The fuel injected in the first half of the intake stroke is injected toward the top surfaces of the pistons and cavities provided in the top surfaces for a certain period while the pistons are still at a high position after top dead center so as to avoid the fuel adhering to the cylinder walls. Consideration is given so that the spray of fuel is vaporized and sufficiently mixed with air during the relatively long period from the intake stroke to the compression stroke.
While not disclosed in the above-mentioned prior art, even in a direct cylinder injection type spark ignition engine, the exhaust is purified by a catalytic converter provided in the exhaust passage in the same way as with a usual spark ignition engine.
While not limited to direct cylinder injection type spark ignition engines, in general when a catalytic converter is used, if the temperature of the catalyst is low and does not reach the activation temperature, such as at the time of cold starts, the hydrocarbons, carbon oxides, etc. are not purified, but are discharged to the atmosphere, so to swiftly raise the purification efficiency, it has been proposed in the past to provide means which provides a heater in the exhaust pipe to heat the catalyst etc., but these are relatively troublesome and induce higher costs.
In particular, in an direct cylinder injection type spark ignition engine, as mentioned earlier, the engine is often run under a state of a higher air richness compared with a usual spark ignition engine, so the exhaust temperature tends to fall due to combustion of a lean air-fuel mixture and even after starting the temperature of the catalyst of the catalytic converter does not easily rise to the activation temperature, so the state of retarded exhaust purification continues for a long period and there is a danger of discharge of the unpurified exhaust.